The spec sheet sees a number of incremental bumps. Qualcomm's octa-core Snapdragon 810 CPU is doing the heavy lifting here, supported by 3GB of RAM, and the M9 also sees the welcome addition of Android 5.0 Lollipop, although the presence of HTC's Sense skin means users won't see such a dramatic aesthetic change between 4.4 and 5.0. The battery capacity has also been bumped up from 2,600mAh to 2,840mAh, which, when combined with the new Snapdragon, will hopefully see the M9 last a full day with ease.
Perhaps the biggest change comes in the form of a new rear-facing camera unit. HTC called an abrupt stop to the megapixel wars with the introduction of its UltraPixel tech, which used large, less densely packed pixels that absorbed more light. That's all gone this time around, as there's now a more conventional 20-megapixel sensor set in a square camera unit at the rear of the phone. HTC is promising big things from this camera, touting its f/2.2, 27.8mm lens and, thanks to the bump in pixel count, 4K video recording. It's not abandoning UltraPixel entirely, however, as it's using that technology for its front-facing camera.
All told, the M9 looks like a good, if incremental, upgrade over its predecessors (especially if you're coming from the original HTC One). We'll get back to you with some more detailed impressions as soon as possible.
Jon Fingas contributed to this report.
Update: We've now got a full preview of the HTC One M9 right here.